Electric ignition system using a ptc ignitor as a sensing means

ABSTRACT

An electric ignition system for a fuel burner including an electrical resistance-type ignitor, wherein the resistance of the ignitor has a positive temperature coefficient and is connected in series with a ballast relay in parallel with the relay contacts and an electrically operated fuel valve, the latter being in the burner fuel supply line; the voltage drop of the energizing circuit for the valve and the ignitor varies according to the temperature of the ignitor for controlling operation of the burner supply line.

United States Patent [72] Inventors JamesR.Willson Garden Grove; Hugh 1. Tyler, Santa Ana, both of, Calif. [21] Appl. No. 808.840 [22] Filed Mar. 20, 1969 (45] Patented July 20, 1971 [73] Assignec Robershaw Controls Company Richmond, Va.

[54] ELECTRIC IGNITION SYSTEM USING A PTC Primary Examiner-Frederick L. Matteson Assistant Examiner-Robert A. Dua

Attorneys-Auzville Jackson, Jr., Robert L. Marben and Anthony A. O'Brien IGNITOR AS A SENSING MEANS 26 Claims, 3 Drawing Figs.

U-S. ABSTRACT: An electric ignition system for a fuel burner in- 317/98 eluding an electrical resistance-type ignitor, wherein the re- Cl. no sistance of the has a positive temperature coeffi ient [50] Field of Search 317/98, 79; d i connected in series with a ballast re|ay in parallel with 431/66 74 the relay contacts and an electrically operated fuel valve, the latter being in the burner fuel supply line; the voltage drop of [56] References (mad the energizing circuit for the valve and the ignitor varies ac- UNITED STATES PATENTS cording to the temperature of the ignitor for controlling R'e. 22,554 10/1944 Weber 431/66 operation of the burner supply line. I

I6 1 l2 s J 24 FUEL FUEL BURNER INPUT G VALVE 28 PATENTED JULZOISYI 3594107 FIG. I

22 2 |6 1 I L j 24 j lo] G9 I V FUEL FUEL BURNER INPUT 7 VALVE 26 j 26 28 FIG. 2 I 36 IGNITOR v TEMPERATURE VOLTAGE DROP ACROSS IGNITOR INVENTORS, James R. Willson TIME Hugh J. Tyler awkwvm ATTORNEYS BACKGROUND OF THE INVENTION The present invention relates to electric ignition systems for fuel burners and, more particularly, to an electric ignition system wherein an electric ignitor element having a positive temperature coefficient of resistance is utilized as a fuel ignitor and, at the same time, as a temperature-sensing element for the control of fuel flow to the system burner.

Recent discoveries of various electric ignitor materials having desirable characteristics for commercial use have made electric ignition systems employing such devices increasingly important. One of the many basic goals sought after by en gineers in this field is the provision of a circuit which is relatively simple, has few components, and will-fail-safe under abnormal conditions.

Typical of such devices are circuits which employ a heatsensing element to monitor the burner flame. The heat-sensing element is very often either a thermistor to detect heat directly or a photoelectric light detector to sense the light emitted by the burner flame. The electrical control circuitry associated with these elements, as well as the elements themselves, are quite complex and often operate with considerable lag time which, in certain circumstances, can create a dangerously explosive condition in the immediate burner area.

Different attempts to reach the aforementioned goal and to solve its associated problems have resulted in circuits which are only partially satisfactory due to either their complexity, their difficulty of production, or their lack of sufficient safety characteristics.

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to construct an electric ignition system having all the advantages of similarly employed prior art systems but utilizing fewer components than the components essential to such prior art systems.

Another object of this invention is to provide an electric ignition system with an electric ignitor element acting as both an igniting means and a heat-sensing means.

This invention has a further object in the provision of an electric ignition system having increased fail-safe characteristics.

An additional object of the present invention is the provision of simplified burner flame-proving circuitry for use with an electricignitor.

A still further object of this invention is to construct an electric ignition system having redundant safety characteristics for preventing raw fuel leakage.

An advantage of the invention is the provision of a simple and reliable electric ignitor system having few parts and inherent safety characteristics.

The present invention is summarized in that an electric ignition system for a fuel burner includes an electrical power source coupled to an electric ballast device and an electric ignitor, located adjacent the fuel burner, is coupled to the power source and the ballast device and has a positive temperature coefficient of resistance; an electrically operated fuel valve for the fuel burner is electrically coupled with the electric ignitor to form, an energizing circuit where the electrically operated valve is closed in the absence of electrical energization and opened upon energization by a. voltage having at least a particular value, and the energizing circuit produces a voltage drop which is directly proportional to the temperature of the ignitor and which has a value equal to the above-mentioned particular value when the ignitor is above fuel igniting temperatures. 1

These and other objects and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiments of the invention when considered in conjunction with the accompanyingdrawing.

, 2 BRIEF DESCRIPTION OF THE DRAWING FIG. I shows a schematic circuit diagram of an ignition system embodying the present invention;

' FIG. 2 shows a schematic circuit diagram of a modification of the circuit of FIG. I incorporating flame-proving circuitry; and

' FIG. 3 shows a graph illustrating the principles of operation of the circuits of FIGS. 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. I, which illustrates a preferred embodiment of the invention, shows an electric ignitor 10, having a positive temperature coefficient of resistance, connected in series with the energization winding of an electromagnetic relay 12, a ballast resistor 14, and a switch 16. The entire series circuit is connected to a source of electrical power, illustrated for simplicity by lines I8. Ballast resistor 14 and the winding of relay [2 may be replaced by a relay having a ballast winding, if desired, and are illustrated separately merely for the sake of clarity.

Connected in parallel with ignitor 10 is an additional series circuit comprising an electrically operated fuel valve 20, a switch 22, and a set of normally open relay contacts 24 which are open (as shown) when relay 12 is not energized and are responsive to energization of relay 12 to move to a closed position. The electrically operated fuel valve 20 may be of any well-known type, and may utilize any conventional type actuator, such as a heat motor, an electrom'agnet, etc. Accordingly, the fuel valve 20 is constructed and designed so as to be open only when energized by a voltage having a value which is greater than orequal to V (to be more fully explained with reference to FIG. 3). When fuel valve 20 is open, fuel .will flow via conduit 26 from a fuel source (not shown) through the fuel valve 20 to the fuel burner 28 located within igniting proximity of electric ignitor 10.

Referring now to FIG. 2, wherein like reference numerals are used to refer to similar components as used in FIG. 1', ignitor I0 is coupled in series with a hot-wire ballast relay 30, switch 16, and a positive temperature coefficient thermistor 32 across power source 18. Hot-wire ballast relay 30 comprises a ballast arm 34 which acts as a series ballast resistor and expends when self-heated by current flowing therethrough to close a set of contacts 36. Contacts 36 are connected in series with switch'22 and the energization winding of electromagnetic relay 38. Relay 38 is magnetically coupled to normally open contacts 24 and a set of normally closed contacts 40 which are connected directly across thermistor 32. Relay 38, when energized, closes contacts 24 while simultaneously opening contacts 40.

The circuits illustrated in FIGS. 1 and 2 operate in a similar manner, in accordance with the principles of operation of the invention. Both circuits may be utilized in any of various fuel burner systems, however, their operation will be explained herein in the context of a central heating furnace system. In such a system, switch 26 might correspond to either the main power switch, if it were desired that the ignitor be constantly energized, or the system thermostat. If switch 16, in a particular installation, corresponds to the system thermostat, switch 22 is not necessary and can be eliminated. It is noted that switches I6-and- 22 can beoperated either manually or automatically in accordance with the particular requirements of the installation. Furthermore, directly operated, or indirectly thermostat where the switching element is responsive to a thermally sensitive device which monitors the temperature of the surrounding area, and are shown as single-pole singlethrow switches for illustrative purposes only.

It is also noted that resistor 14 need not be a ballast resistor, but may be of any suitable type such as a conventional fixed resistor; however, the used a ballast resistor, which operates in a conventional manner as a line-voltage-compensating device, is. preferred since the ballast operation of theelectric ignition system of the present invention by preventing power line voltage fluctuations from adversely afi'ectingsystem operation.

the switches may be either operated as in a conventional resistor enhances the In explaining the operation of the circuit of FIG. 1, a typical installation of the ignition system for use with a central heating furnace will be used wherein switch 16 corresponds to a main power-switch, and switc'h'22 corresponds to a thermostatically operated switch. When no demand for heat exists and the power switch is off, the system will be at rest, as illustrated. When the thermostat element (not shown) senses a demand for heat, switch 22 will close, preparing the fuel valve 20 for energizatiodAM is point, closure of main power switch 16 will cause curreiit to flow from power source 18 through the series circuit including ignitor 10, relay winding 12 and ballast resistor 14. The relay '12 will therefore be energized causing contacts 24 to close thereby placing the electrically operated valve 20 in parallel with the ignitor. Since the ignitor is cold, its resistance, which increases in direct proportion with its temperature,- will have a relatively low value with the ignitor and the remaining resistance of the series circuit acting as a voltage divider. Thus, the voltage drop across the ignitor initially will besmall. Since the fuel valve 20 is connected in parallel with the ignitor, the voltage drop across the ignitor is directly applied to the valve for energization thereof. As the ignitor increases in-temperature, its resistance will correspondingly increase as will the voltage drop thereacross. Thus, the amplitude of the'voltage drop across igniter [0 acts as a control signal which is responsive to ignitor temperature to preclude raw fuel leakage from the system.

To further clarify the operation of the system, reference will now be made to FIG. 3, wherein there is shown a curve 42 which illustrates the variation of ignitor temperature with time when the ignitor is initially energized. The curve also illustrates the variation in the voltage drop across the ignitor as its temperature varies. In the sequence of events described in the preceding paragraph, time t in the graph corresponds to the time when the main power switch 16 is turned on. As seen in FIG. 3, after the power switch 16 is closed the ignitor temperature and the voltage drop thereacross both increase to the point where fuel igniting temperature, indicated as T, is reached. At that time, however, the voltage drop, which is the potential appearing across the fuel valve, will be below V which is the minimum voltage needed to effectuate opening of the valve 20. Thus, the fuel valve will only permit fuel to flow to the system bume'rwhen the temperature of the electric ignitor l0 is above thatvalue T which is necessary for ignition. The ignitor 10, therefore, acts as both an igniting element and a heat sensing device to prevent raw fuel leakage.

The circuit also provides effective fail-safe characteristics since a failure in any electrical component which produces either a short circuit or an open circuit will rapidly deenergize the fuel valve to shut oh the flow of raw fuel to the burner. More particularly, if the ignitor opens, the current through that the increasing temperature of the ignitor produces an increasing voltage drop across the relay coil 38 which will be sufficiently great to cause its actuation only if the ignitor temperature is above fuel-igniting temperatures.

It is also pointed out that arm 34 of hot-wire relay need not be a ballast arm, but may be of any suitable type such as a conventional heat motor operator which expands when heated to close contacts 36. However, as is the case with resistor 14, the use of a ballast device is preferred since the line voltage compensating characteristics thereof serve to enhance the operation of the electric ignition system of the present invention by preventing powerline voltage fluctuations from adversely affecting system operation.

In the embodiment shown in FIG. 2, an additional safety function is provided. When relay 38 is energized, normally closed contacts 40 are opened thereby connecting thermistor 32 in series with ignitor l0. Thermistor 32 has a positive temperature coefficient and will have a sufficiently high value of resistance to reduce the voltage drop across relay coil 38 to a value which is insufficient to maintain its actuation unless a burner flame exists. When the flame is present, it will heat the ignitor causing its temperature, and therefore its resistance, to increase. to the point'where, even with the increased resistance of thermistor 32 in the series circuit, a sufficient voltage drop will be provided to energize the relay 38. If the burner flame relay 12 will cease,opening contacts 24 and removing ener- I gization from the fuel valve. If, on the other hand, the ignitor shorts, the potential across the parallel coupled valve will become zero causing deenergization thereoflt is additionally pointed out that relay 12 can be designed to be current sensitive so that it will only operate when sufiicient current is flowing therethrough to assure heating of the ignitor to fuel-igniting temperatures. Thus, by making the relay current sensitive and the fuel valve voltage sensitive, the device inherently possesses redundant safety featuresnot heretofore provided.

The operation of the circuit shown in FIG. 2 is similar to that of the system of FIG. 1. When power switch 16 is closed, 7

current will flow fromsource 18 through the ballast arm 34 of hot-wire ballast relay 30, ignitor l0 and around resistor 32 via contacts 40. Current flowing through ballast arm 34 causes self-heating which produces expansion thereby closing contacts 36. If a demand for heat exists, thermostatically operated switch 22 will be closed so that current will be permitted to flow through the energization winding of relay 38 which will then be in parallel with the ignitor 10. In the embodiment of FIG. 2, relay 38 serves as the voltage-sensing element and, when actuated, closescontacts 24 thereby coupling valve 20 .to the power source for energization. The operation of the circuit thus follows in a similar manner as the circuit of FIG. 1, in

should go out, however, the resistance of the ignitor will decrease and the potential across the relay coil will be insufficient to maintain fuel flow and therefore raw fuel leakage will be prevented. It is noted that the positive temperature coefficient characteristic of thermistor 32 provides a delay time for the ignitor to come to fuel igniting temperatures and establish a burner flame before the flame-proving feature is enabled.

In summary, this invention provides a simple, yet reliable, electric ignition system wherein the ignitor acts as a fuel-igniting device as well as a rapid acting heat-sensing element. In this manner, the fail-safe features of such a system are increased while the number of components required'is reduced. This invention therefore makes electric ignition systems available for many diverse applications requiring high-safety and low-production cost. I

Inasmuch as'the present invention is subject to many variations, modifications and changes in detail, it is intended that all matter contained in the foregoing description or shown in the accompanying drawing shall be interpreted as illustrative in nature and not in a limiting sense.

' What we claim is:

I. In an electric. ignition system for a fuel burner, the combination comprising an electrical power source,

control means coupled to said power source,

an electric ignitor adapted to be located adjacent the burner and coupled to said power source and said control means, said ignitor having a positive temperature coefficient of resistance,

valve means adapted to control a flow of fuel to the burner and being electrically coupled with said electric ignitor to form an energizing circuit,

said valve means being closed in the absence of electrical energization and being opened upon energization by a voltage across said energizing circuit having at least a particular value, and

said energizing circuit having a voltage drop which is directly proportional to the temperature of said ignitor and which has a value equal to said particular value when said ignitor is above fuel-igniting temperatures.

2. The invention as recited in claim l wherein said control means includes current-sensing means for sensing a flow of current to said energizing circuit; and including switch means responsive to said current-sensing means and coupled in series with said valve means for disabling said valve means when said flow of current is below a predetermined level.

3. The invention as recited in claim 2 wherein said currentsensing means includes a ballast relay energization winding.

4. The invention as recited in claim 2 wherein said currentsensing means includes a relay energization winding, and said control means includes a ballast resistor coupled in series with said relay energization winding.

5. The invention as recited in claim 2 wherein said currentsensing means includes a hot-wire ballast relay, and said valve means includes an electrically operated valve and an electromagnetic relay having an energization winding and at least a normally open set of contacts coupled in series with said electrically operated valve across said electrical power source, said contacts closing in response to energization of said electromagnetic relay.

6. The invention as recited in claim 5 including means enabling burner flame proving coupled to said electromagnetic relay and said energizing circuit for reducing the voltage drop across said energizing circuit to a value below said particular value upon burner flame outage.

7. The invention as recited in claim 6 wherein said burner flame-proving enabling means includes a thermistor having a positive temperature coefficient coupled in series with said energizing circuit and a normally closed set of contacts coupled in parallel with said thermistor, said contacts opening in response to energization of said electromagnetic relay, and said electric ignitor being maintained above fuel-igniting temperatures in the absence of burner flame outage.

8. An electric ignition system for a fuel burner comprising an electrical power source,

an electric ignitor coupled to said power source and having a voltage drop thereacross which increases with temperature and has a particular value when said ignitor is at fueligniting temperatures,

an electrically operated valve for controlling a flow of fuel to the fuel burner,

control means coupled in series with said electric ignitor for providing a constant load and for sensing a flow of current through said ignitor, switch means coupled with said electric ignitor by said control means when a flow of current is sensed thereby, and

said switch means coupling said electrically operated valve to said power source for energization when the voltage drop across said electric ignitor has a value greater than its said particular value.

9. The invention as recited in claim 8 wherein said control means includes a hot-wire ballast relay and wherein said switch means includes an electromagnetic relay having an energization winding and at least a normally open set of contacts coupled in series with said electrically operated valve across said electrical power source, said contacts closing in response to energization of said electromagnetic relay.

10. The invention as'recited in claim 9 including means enabling burner flame proving coupled to said electromagnetic relay and said electric ignitor for reducing the voltage drop across said electric ignitor to a value below said particular value upon burner flame outage.

11. The invention as recited in claim 10 wherein said burner flame-proving enabling means includes a thermistor having a positive temperature coefficient coupled in series with said electric ignitor and a normally closed set of contacts coupled in parallel with said thermistor, said contacts opening in response to energization of said electromagnetic relay and said electric ignitor being maintainedabove fuel-igniting temperatures in the absence of burner flame outage.

12. In an electric ignition system for a fuel burner, the combination comprising means for providing electrical power,

an electric ignitor adapted to be located adjacent the burner and coupled to said power means,

said ignitor having a positive temperature coefficient of resistance,

an electrically operated valve adapted to control a flow of fuel to the burner and being directly coupled with said electric ignitor to form an energizing circuit,

said valve being closed in the absence of electrical energization and being opened upon energization by a signal having at least a particular amplitude, and

said energizing circuit producing a signal responsive to the temperature of said ignitor having an amplitude equal to said particular amplitude when said ignitor is above fueligniting temperatures.

13. An electric ignition system for a fuel burner comprising input means adapted to be connected with a source of electricity,

an electric ignitor coupled with said input means and adapted to be disposed within igniting proximity of the burner, said electric ignitor having a positive temperature coefficient of resistance,

valve means in direct electrical circuit with said electric ignitor and adapted to control a flow of fuel to the burner, said valve means being operated in accordance with the resistance of said electric ignitor.

14. The invention as recited in claim 13 wherein said elec- 1 tric ignitor exhibits a high resistance at fuel-igniting temperatures, and said valve means is opened in response to said high resistance whereby said valve means will open after said electric ignitor reaches ignition temperature.

15. The invention as recited in claim 14 further including flame proving means coupled to said valve means and said electric ignitor for closing said valve means upon burner flame outage.

16. The invention as recited in claim 15 wherein said flame proving means comprises a thermistor having a positive temperature coefficient.

17. The invention as recited in claim 14 wherein said valve means is electrically connected in parallel circuit with said electric ignitor.

18. An electric ignition system for a fuel burner comprising input means adapted to be connected with a source of electricity,

an electric ignitor connected with said input means and adapted to be disposed within igniting proximity of the burner, said electric ignitor having a positive temperature coefficient of resistance such that current through said electric ignitor and voltage across said electric ignitor are dependent upon ignitor temperature,

valve means adapted to control a flow of fuel to the burner,

current-sensing means connected with said electric ignitor for sensing the amount of current flowing through said electric ignitor,

voltage-sensing means connected with said electric ignitor for sensing the amplitude of the voltage across said electric ignitor,

first control means coupled with said voltage sensing means and said valve means to control said valve means in response to the amplitude of the voltage across said electric ignitor, and

second control means coupled with said current-sensing means to control said valve means in response to the amount of current flowing through said electric ignitor.

19. The invention as recited in claim 18 wherein said current-sensing means is in series with said electric ignitor.

20. The invention as recited in claim 19 wherein said second control means comprises a switch responsive to said currentsensing means.

21. The invention as recited in claim 18 wherein said voltage-sensing means is connected in parallel with said electric ignitor.

22. The invention as recited in claim 21 wherein said voltage-sensing means comprises an electromagnetic coil.

23. The invention as recited in claim 22 wherein said first control means comprises a switch responsive to said voltagesensing means.

24. An ignition system for a fuel burner comprising input means adapted to be connected to a source of electricity,

said valve means to control said valve means in response to the amount of current flowing through said electric ignitor whereby said valve means is responsive to the temperature of said electric ignitor. 25. The invention as recited in claim 24 wherein said current-sensing means is in series with said electric ignitor.

26. The invention as recited in claim 25 wherein said control means includes a switch connected in series with said valve means and said input means. 

1. In an electric ignition system for a fuel burner, the combination comprising an electrical power source, control means coupled to said power source, an electric ignitor adapted to be located adjacent the burner and coupled to said power source and said control means, said ignitor having a positive temperature coefficient of resistance, valve means adapted to control a flow of fuel to the burner and being electrically coupled with said electric ignitor to form an energizing circuit, said valve means being closed in the absence of electrical energization and being opened upon energization by a voltage across said energizing circuit having at least a particular value, and said energizing circuit having a voltage drop which is directly proportional to the temperature of said ignitor and which has a value equal to said particular value when said ignitor is above fuel-igniting temperatures.
 2. The invention as recited in claim 1 wherein said control means includes current-sensing means for sensing a flow of current to said energizing circuit; and including switch means responsive to said current-sensing means and coupled in series with said valve means for disabling said valve means when said flow of current is below a predetermined level.
 3. The invention as recited in claim 2 wherein said current-sensing means includes a ballast relay energization winding.
 4. The invention as recited in claim 2 wherein said current-sensing means includes a relay energization winding, and said control means includes a ballast resistor coupled in series with said relay energization winding.
 5. The invention as recited in claim 2 wherein said current-sensing means includes a hot-wire ballast relay, and said valve means includes an electrically operated valve and an electromagnetic relay having an energization winding and at least a normally open set of contacts coupled in series with said electrically operated valve across said electrical power source, said contacts closing in response to energization of said electromagnetic relay.
 6. The invention as recited in claim 5 including means enabling burner flame proving coupled to said electromagnetic relay and said energizing circuit for reducing the voltage drop across said energizing circuit to a value below said particular value upon burner flame outage.
 7. The invention as recited in claim 6 wherein said burner flame-proving enabling means includes a thermistor having a positive temperature coefficient coupled in series with said energizing circuit and a normally closed set of contacts coupled in parallel with said thermistor, said contacts opening in response to energization of said electromagnetic relay, and said electric ignitor being maintained above fuel-igniting temperatures in the absence of burner flame outage.
 8. An electric ignition system for a fuel burner comprising an electrical power source, an electric ignitor coupled to said power source and having a voltage drop thereacross which increases with temperature and has a particular value when said ignitor is at fuel-igniting temperatures, an electrically operated valve for controlling a flow of fuel to the fuel burner, control means coupled in series with said electric ignitor for providing a constant load and for sensing a flow of current through said ignitor, switch means coupled with said electric ignitor by said control means when a flow of current is sensed thereby, and said switch means coupling said electrically operated valve to said power source for energization when the voltage drop across said electric ignitor has a value greater than its said particular value.
 9. The invention as recited in claim 8 wherein said control means includes a hot-wire ballast relay and wherein said switch means includes an electromagnetic relay having an energization winding and at least a normally open set of contacts coupled in series with said electrically operated valve across said electrical power source, said contacts closing in response to energization of said electromagnetic relay.
 10. The invention as recited in claim 9 including means enabling burner flame proving coupled to said electromagnetic relay and said electric ignitor for reducing the voltage drop across said electric ignitor to a value below said particular value upon burner flame outage.
 11. The invention as recited in claim 10 wherein said burner flame-proving enabling means includes a thermistor having a positive temperature coefficient coupled in series with said electric ignitor and a normally closed set of contacts coupled in parallel with said thermistor, said contacts opening in response to energization of said electromagnetic relay and said electric ignitor being maintained above fuel-igniting temperatures in the absence of burner flame outage.
 12. In an electric ignition system for a fuel burner, the combination comprising means for providing electrical power, an electric ignitor adapted to be located adjacent the burner and coupled to said power means, said ignitor having a positive temperature coefficient of resistance, an electrically operated valve adapted to control a flow of fuel to the burner and being directly coupled with said electric ignitor to form an energizing circuit, said valve being closed in the absence of electrical energization and being opened upon energization by a signal having at least a particular amplitude, and said energizing circuit producing a signal responsive to the temperature of said ignitor having an amplitude equal to said particular amplitude when said ignitor is above fuel-igniting temperatures.
 13. An electric ignition system for a fuel burner comprising input means adapted to be connected with a source of electricity, an electric ignitor coupled with said input means and adapted to be disposed within igniting proximity of the burner, said electric ignitor having a positive temperature coefficient of resistance, valve means in direct electrical circuit with said electric ignitor and adapted to control a flow of fuel to the burner, said valve means being operated in accordance with the resistance of said electric ignitor.
 14. The invention as recited in claim 13 wherein said electric ignitor exhibits a high resistance at fuel-igniting temperatures, and said valve means is opened in response to said high resistance whereby said valve means will open after said electric ignitor reaches ignition temperature.
 15. The invention as recited in claim 14 further including flame proving means coupled to said valve means and said electric ignitor for closing said valve means upon burner flame outage.
 16. The invention as recited in claim 15 wherein said flame proving means comprises a thermistor having a positive temperature coefficient.
 17. The invention as recited in claim 14 wherein said valve means is electrically connected in parallel circuit with said electric ignitor.
 18. An electric ignition system for a fuel burner comprising input means adapted to be connected with a source of electricity, an electric ignitor connected with said input means and adapted to be disposed within igniting proximity of the burner, said electric ignitor having a positive temperature coefficient of resistance such that current through said electric ignitor and voltage across said electric ignitor are dependent upon ignitor temperaTure, valve means adapted to control a flow of fuel to the burner, current-sensing means connected with said electric ignitor for sensing the amount of current flowing through said electric ignitor, voltage-sensing means connected with said electric ignitor for sensing the amplitude of the voltage across said electric ignitor, first control means coupled with said voltage sensing means and said valve means to control said valve means in response to the amplitude of the voltage across said electric ignitor, and second control means coupled with said current-sensing means to control said valve means in response to the amount of current flowing through said electric ignitor.
 19. The invention as recited in claim 18 wherein said current-sensing means is in series with said electric ignitor.
 20. The invention as recited in claim 19 wherein said second control means comprises a switch responsive to said current-sensing means.
 21. The invention as recited in claim 18 wherein said voltage-sensing means is connected in parallel with said electric ignitor.
 22. The invention as recited in claim 21 wherein said voltage-sensing means comprises an electromagnetic coil.
 23. The invention as recited in claim 22 wherein said first control means comprises a switch responsive to said voltage-sensing means.
 24. An ignition system for a fuel burner comprising input means adapted to be connected to a source of electricity, an electric ignitor connected with said input means and adapted to be positioned in igniting proximity to the burner, said electric ignitor having a positive temperature coefficient of resistance such that the amount of current through said electric ignitor is dependent upon ignitor temperature, valve means adapted to control a flow of fuel to the burner, current-sensing means connected with said electric ignitor for sensing the amount of current flowing through said electric ignitor, and control means coupled with said current-sensing means and said valve means to control said valve means in response to the amount of current flowing through said electric ignitor whereby said valve means is responsive to the temperature of said electric ignitor.
 25. The invention as recited in claim 24 wherein said current-sensing means is in series with said electric ignitor.
 26. The invention as recited in claim 25 wherein said control means includes a switch connected in series with said valve means and said input means. 